WARNING! Spoilers for The Haunting of Bly Manor ahead.
Throughout The Haunting of Bly Manor, Hannah Grose (T'Nia Miller) gets lost in thought and daydreams. When episode 5, "Altar of the Dead", revealed that she has been dead since Danielle "Dani" Clayton (Victoria Pedretti) arrived at the manor, the truth behind her ghostly presence began to unfold. Mrs. Grose is unlike any spirit on the grounds because she did not forget herself and didn't even realize she was dead, but what's the real reason that she didn't forget?
The Haunting of Bly Manor begins as a ghost story and transforms into a love story as it nears its conclusion. Mike Flanagan's recent addition to The Haunting series is loosely based on Henry James's 1898 novella The Turn of the Screw as well as several of his other ghost stories. It follows Dani, who has recently taken up the position of being the au pair to Miles and Flora Wingrave, two young children who lost their parents in a tragic accident years prior. While staying in the manor, she quickly discovers that there are ghosts hidden in the darkest corners as well as in broad daylight. It isn't obvious to the young au pair that the friendly housekeeper, Mrs. Grose, is a spirit until the very end, mainly due to the fact that - unlike the other ghosts - she refused to forget herself.
The series provides subtle hints at the big reveal that Hannah Grose is dead but viewers are not aware of this until episode 5. Peter Quint (Oliver Jackson-Cohen) possesses Miles and uses him to push her down a well, which happened moments before Dani arrived at Bly. Though Hannah went along like nothing happened, her actions became increasingly suspect until the big reveal that she's been dead the whole time. However, that still doesn't explain how she's seemingly immune to the same, tragic forgetting that the other ghosts all eventually experience. By the time episode 7, "The Two Faces, Part Two", takes place, the real reason Mrs. Grose didn't forget herself is unveiled.
Hannah Grose has been wandering the grounds of Bly Manor believing that she is still alive and living her life like nothing's changed. It's her denial that causes her to remain somewhat attached to the realm of the living. Episode 5 also showcased that Mrs. Grose did not have time to process her death because Dani arrived just moments after her fatal fall took place. Two other recently dead spirits, Peter Quint and Rebecca Jessel, had already begun to forget themselves, but this may be due to the fact that they saw their dead bodies and subsequently got the opportunity to mourn and process their deaths. On the other hand, Mrs. Grose didn't have this opportunity, which may have propelled her into some form of blissful ignorance instead.
Despite the fact that she may not be overtly aware that she is dead, the four candles she lights in the chapel represent the people she's lost: Rebecca Jessel, Miles and Flora's parents, and herself. Mrs. Grose is subconsciously aware that she is dead, but uses her memories and love for Owen to anchor her to reality. In "Altar of the Dead", she repeatedly returns to the moment she met Owen to interview him for a cooking position at the manor. Mrs. Grose continues to return to this moment because it helps her remember who and where she is with the phrase, "You are Hannah Grose. The year is 1987. You are at Bly. Miles is ten. Flora is eight."
When Hannah Grose died, she did not have the time to process the event and, even though she was always subconsciously aware of it, she utilized her memories to keep her anchored to the realm of the living. She never wandered the halls as a vengeful, heartbroken, or resentful spirit. Instead, Mrs. Grose maintained her positive outlook, held onto the emotions she felt while she was alive, and continued to live her life as though she never fell down the well at the beginning of The Haunting of Bly Manor.